The Giant Sea Snake is encountered at the end of the Royal Trouble quest. It is not as strong as its combat level suggests, but an antipoison potion, preferably super, is vital, as the snake is poisonous with damage starting in the nineties. It fights with melee up close and ranged from a distance, but it cannot detect the use of prayers. Like its offspring, it is vulnerable to slashing weapons.
The best way to defeat the snake is to attack it up close with melee, using a slashing weapon such as a Scimitar or an Abyssal whip to exploit its weakness. This way, it will not use its ranged attack, so players can freely use Protect from Melee to reduce the majority of the damage. Other factors such as what food to bring and what armour and weapon to use are left to the player's decision. It is recommended to bring at least lobsters or swordfish.